----- Setting Jet Transport AIR file Parameters -----
Ron Freimuth 10 Aug, 2000
Rev: 8/23/00, 9/22/00, 9/28/00, 2/15/01
These comments only cover a few things involved, including some
newer approaches and information.
Moments of Inertia:
One should set the three main MoI's close to the real values.
Often they are way too high. They affect attitude rates and damping.
CFSCalculator implements values that Roskam gives for typical AC,
these numbers appear to be for typically loaded AC, rather than empty.
Thus, they would include fuel. But, setting them on Empty Weight
should give reasonable values as far as changes in fuel goes. However,
the payload will increase Pitch MoI significantly, unless the payload is
DISTRIBUTED along the fuselage by adding entries to AIRCRAFT.CFG, the
Pitch MoI will tend to be too low. Thus, one might increase Pitch MoI
above the value calculated for an empty AC from Roskam's formulas (or,
the actual empty value).
One might also use values from similar, default AIR files, but
multiply them by:
[(Wing Span)/(Wing Span, default)]^2 * [Weight/(Weight, default)]
Perhaps using the ratio of Fuselage Lengths for the Pitch MoI factor,
and the ratio of (Span + Length)'s for Yaw MoI factor.
Try to keep the three in typical proportion. Pitch MoI might be two
times the value of Roll MoI, and Yaw MoI is NO higher than the sum of
Roll MoI + Pitch MoI.
The fourth MoI is a Cross Moment of Inertia. It is due to the
asymmetry in the horizontal plane of the vertical stabilizer above the
roll axis. This is often 0, sometimes 1000 or so for small AC, but about
2% of Yaw MoI in some AC, such as the 747. Perhaps higher in AC with an
engine in the tail or a 'T' tail. I don't think it has much effect so set
it to zero or leave propional to the previous value unless you have data
to set it otherwise.
Stability Derivatives:
These are labeled in sec 1101 and include Drags, Lifts, Couplings,
Control Authority, etc. They are NORMALIZED. That is, they should be
roughly the same for all AIR files. Most have some effect on flight
stability (damping). Many are fairly obvious (Help in Aired.ini may
explain more), but some have never been tested. Many are more critical
for CFS fighters.
If one has a well performing AIR file, simply changing the MoI's,
Wing Area and Chord, "Control Surfaces" (areas), Landing Gear parameters,
Zero Fuel Weight, and Number of Engines/Thrust should result in a model
which has appropriate dynamics, considering the change in size. Changing
Thrust in proportion to weight will give about the same climb and cruise
performance. Adjust the 'Turbine Intake Area" to tweak the high altitude
climb and maximum speed (the Mach number gets higher here).
Drags:
'Drag - Zero Lift' is relative to Wing Area. Typically, it ranges
from '33' to '60' as entered in Airedit. '40' is about right for typical
jet transports. The true value is 1/2048 this value, so if you have a
real number, multiply it by 2048 for the integer in the AIR file.
Mach Drag is significant above Mach 0.8 or so, and assuming 'Drag -
Zero Lift' is reasonable is the main thing to adjust to set total drag
and PPH (range) at cruise altitude and Mach number. I ended up with
'14' at Mach 0.8 in an A310, since it cruises a bit slower than some
jets. This might be as low as '4' in jets that cruise at Mach 0.84,
increasing to 55-80 at Mach 1.0. The Value at Mach 0.8 is most critical
for aircraft that cruise around Mach 0.82. The values add directly to
'Drag - Zero Lift'.
Even though Mach effects don't set in until about Mach 0.7, the Mach
Drag Table can be used to set a slowly increasing drag above Mach 0.3 or
so. This might be 10% to 20% of 'Drag - Zero Lift' at Mach 0.6.
TBL 401 appears to multiply Cl in TBL 403 as Mach Number varies.
The default is about 1.38 at Mach 1.0, but dropping that to 1.20 or less
and lower values proportionally seems to increase Induced Drag at cruse
speeds, compared to the default. Thus, it affects climb and cruise fuel
flow at full fuel. Slower AC have it at 1.0 for all Mach numbers.
TBL 401 can be set to account for rapid decreases in Cl just beyond
the rated Mach speeds.
In the 1204 Wing Section, the 'Induced Drag Coefficient' can be
calculated from the Aspect Ratio and assuming an 'e' of about 0.8.
This has the greatest effect on glide performance, most AC run 3000 -
4000. Many AIR files have it set way too low.
Setting 'Winglets' to True appears to reduce Induced Drag by 20%,
which is too much. If you enable them you may need to increase the
'Induced Drag Constant' as much as 17%. Which would leave an overall
3% decrease in drag with the Winglets.
'Twist' of 0.0 to -1.0 deg seems the default, and it's probably
better not to go outside that range unless you know the real value.
It appears to add a constant value to induced drag. It also changes
the cruise pitch by about 40% of the setting. Changing Twist from
0.0 to -1.0 degree would increase cruise pitch by 0.4 degrees.
'Angle of Incidence' is the angle of the wing chord relative to the
longitudinal axis of the fuselage. It affects the cruise pitch: 0 to 2.0
degrees seems reasonable. However, TBL 403 usually has significant Cl
at AoA=0.0 so a Wing Incidence of 0 is often good. Due to complications
of the Wing/Fuselage interaction the true physical AoI setting may not
be exactly correct for this parameter.
A pitch of 3 degrees or so is typical in cruise for jet transports.
Sec 1101:50 sets the Angle of Attack the minimum drag occurs at.
Setting it to AoA Cl=0.0 in TBL 403 is typical.
Turbines:
In the Turbine Section (1501) the SL Static Thrust parameter is
usually set to the rated SL continuous thrust. This lets the full
throttle thrust exceed the rated value. TOGA sets N1 to 90%, which
sets thrust to the SL rated value in the 777 turbine tables. One
could reduce this thrust parameter by 20% or so if he wished and
still get rated SL thrust for TO with manual throttle control. In
that case you may wish to reduce 'Turbine Inlet Area' parameter to
restore thrust at cruise altitudes.
'Turbine Intake Area' is also in sec 1501. While one can start with
the actual number if he has the data, I've increased this by quite a bit
to at times model the decrease in thrust with Mach Number which occurs
as flight altitude increases. AFTER the drags are correct (which sets top
speed and range) one can change this parameter to limit the available
thrust at typical FL 320 cruise Mach Number. To make climb rate at higher
altitudes realistic (low) and N1/N2 read in the 80's (or, as appropriate).
This is quite critical, a change from '90' to '92' is noticeable.
For modern, high bypass ratio turbines, Intake Area around 1.8 *
Thrust/1000 seems about right (50,000 lb -> 90). For military jets, the
number would be lower -- they don't drop off with altitude as fan jets
do.
N1/N2: These values can be set in the Turbine tables to show the
correct gauge readings. TBL 1506, Thurst vs CN1* can be adjusted
to get appropriate thrust at a given CN1. Drag should not be changed
to get the N1/N2 readings one wants. Rather, the values displayed by
panel gauges should only be adjusted after the thrust and fuel flow
rate are correct. After I have optimised my approach to adjusting N1
(and N2) I'll probably add it to this document. I believe "Turbine
Inlet Area" can be set to to the physical value if the turbine tables
are correctly adjusted.
* CN1/CN2 are 'Corrected N1/N2' The panel gauges indicate N1/N2.
N1=CN1*Sqrt(TAT/288).
Where TAT is Stagnation Air Temperature in degress Kelvin.
C deg = K-273. Note 288 K is 15 C, the standard reference
temperature. Since temperature is low at cruise altitudes,
N1/N2 are about 10% below CN1/CN2 at cruise conditions.
Other:
'Braking Factor' less than the default is usually more realistic.
16,000 - 25,000 (NOT negative). 'Reverse Thrust Throttle Percent' of
-15% to -30% seems appropirate for FS2K AIR files.